The present invention relates to an imaging lens for forming an image on an imaging element such as a CCD sensor and a CMOS sensor. In particular, the present invention relates to an imaging lens suitable for mounting in a relatively small camera such as a cellular phone, a digital still camera, a portable information terminal, a security camera, an onboard camera, and a network camera.
In these years, most models of cellular phones include a camera as standard so as to increase their added values as cellular phones. Furthermore, such integration of digital still cameras in cellular phones has advanced in each year, so now that there is even available a cellular phone, which exhibits optical performances and has other functions that are well competitive to digital still cameras.
An imaging lens for mounting in a cellular phone is strongly required to have a small size for resolution of the imaging element, as well as sufficient optical performances. Conventionally, an imaging lens with a two- or three-lens imaging configuration has been used to ensure sufficient optical performances and a small size thereof in a balanced manner. In each year, however, required optical performances are higher and higher with advancement in attaining higher resolution. Therefore, such imaging lens with a two- or three-lens configuration is not good enough to correct aberrations and it is more difficult to ensure to meet required optical performances.
For this reason, there have been studies in adding another lens, i.e. an imaging lens with a four-lens configuration. For example, an imaging lens described in Patent Reference includes in this order from an object side, a first lens that has a biconvex shape and is positive; a second lens that has a shape of a negative meniscus lens directing a convex surface thereof to the object side; a third lens that has a shape of a positive meniscus lens directing a concave surface thereof to the object side; and a fourth lens that has a biconcave shape and is negative. According to this configuration, the ratio of a focal length of the fourth lens and a focal length of a whole lens system is set in preferred range. Keeping the ratio within the range, it is possible to ensure satisfactory optical performances while restraining increase in total length of the imaging lens.    Patent Reference: Japanese Patent Application Publication No. 2009-20182
According to the conventional imaging lens described in Patent Reference, it is possible to attain relatively satisfactory aberration. Cellular phones themselves, however, have become smaller and have higher functions every year, so that the size required for such imaging lens has been even smaller than before. In case of the lens configuration described in Patent Reference, it is difficult to attain both miniaturization and satisfactory aberration correction, which are required in an imaging lens.
Here, such issue is not specific to an imaging lens for mounting in the cellular phone to attain both miniaturization and satisfactory aberration correction, and it is a common task among imaging lenses for mounting in relatively small cameras such as digital still cameras, portable information terminals, security cameras, onboard cameras, and network cameras.
In view of the problems of the conventional techniques described above, an object of the present invention is to provide an imaging lens that can satisfactorily correct aberration while still attaining miniaturization.